The present invention relates to a device forming a touch screen of the capacitive type intended in particular to cover a visualizing screen of a visualizing display unit such for example as a cathode ray tube.
One knows such devices comprising a plate-shaped transparent dielectric substrate such as glass the front face of which opposite to the visualizing screen comprises a certain number of electrically conducting electrodes insulated from each other so as to form a matrix of keys. These electrodes forming keys are connected to an electronic circuit accommodated in the visualizing unit through the medium of conducting wires deposited on the front face of the substrate and terminating into two rows of contact points spaced from the lower and upper edges, respectively, of the substrate. The conducting wires for the connection of the internal electrodes of the matrix to the electronic circuit are disposed in the spaces between adjacent electrodes. The electrodes and the wires are generally made from indium and tin oxide. When an operator is putting a finger upon one of the electrodes forming keys there occurs a capacity variation influencing the natural frequency of an oscillator associated with this key forming a part of the electronic circuit.
These known devices exhibit the following inconveniences:
an intensive use of the touch screen produces in the course of time a wear of the electrodes forming keys and of the conducting tracks which for the latter may result in an interruption of their continuity. This wear causes failures of the key-forming electrodes and of the tracks involved. This failure occurs pursuant to scratches of the front face of the touch screen which may also interrupt the continuity of the tracks; PA1 a transmission to the electronic circuit of erroneous data often occurs by direct touching of the connecting tracks instead of the electrodes forming keys in particular in the case where these electrodes are of relatively small dimensions; PA1 a destruction of the whole or a part of the electronic circuit associated with the electrodes forming keys by electrostatic discharges upon the screen is to be feared.
Some known devices provide the deposit of an insulating protective layer of SiO.sub.2 or the like upon the front face of the touch screen but this layer is of a thickness limited by the present technique and is wearing out relatively quick so that the inconveniences mentioned hereinabove are found again.